1. Field of the Invention
The present invention relates to semiconductor test fixtures, and, more specifically, to a method for controlling the impedance of signal spring probes in a semiconductor test fixture spring probe array through the placement of ground spring probes.
2. Description of Related Art Including Information Disclosed under 37 CFR 1.97 and 1.98
The semiconductor test industry uses an interface to transfer signals from a device under test (DUT) to a test system. This device typically contains thousands of transistors that are to be tested. The interface between the DUT and the test system comprises a spring probe array that affords a temporary connection between the DUT and the system.
FIG. 1 shows a traditional spring probe array tower with its arrays of spring probes projecting upwards. A typical test setup utilizes a spring probe array with a multitude of spring probes for contacting the DUT. Test signals flow between the test setup and the DUT across the probe connections. Quite often, this device testing requires that the signal impedance be tightly controlled between the DUT and the test system. This is necessary when dealing with high circuit frequencies or when power transfer between the devices must be maximized. Improper impedance can cause reflected signals which interfere with circuit measurements.
The spring probe array includes signal probes and ground probes. FIG. 2 depicts the ground and signal spring probe placement of the traditional spring probe array. Ground probes are spaced appropriately among the signal probes to influence the signal probe impedance. Traditional probe arrays require such a high number of ground probes that a physical limitation is imposed on the possible number of signal probes. This further imposes a limitation on the number of transistors that can be tested.
When a DUT is placed in a test fixture, the spring probe array tower is held in contact with the circuit connections. The test fixture must compress the spring probe array tower sufficiently to establish adequate circuit contact. Test systems with interface compression force limits periodically suffer from lack of sufficient signal spring probes through the interface due to the large number of ground spring probes used to control the impedance of the signals. This is because each spring probe requires some amount of force to compress the probe against the DUT to obtain sufficient contact. The overall test system compression force required is directly proportional to the number of spring probes. In a typical spring probe array, the forces required to adequately compress the multitude of spring probes contacting the DUT can often exceed the test fixture compression force limits.
Accordingly, a need exists for a signal spring probe array that allows a reduced number of ground spring probes to control the impedance of the same or an increased number of signal spring probes. Further, a need exists for a spring probe array that provides a greater number of signal spring probes without exceeding tester compression force limits.